A Galvanic cell type oxygen sensor is generally utilized for detecting an oxygen concentration. The principle of the sensor is based on the fact that a current flowing in a lead-oxygen cell is proportional to the oxygen concentration. When the current from the sensor is fed to a resistor connected to it, a voltage varying linearly with the oxygen concentration appears at both ends of the resistor, as shown in FIG. 1. The oxygen concentration monitor of the present invention is designed to check an output of the Galvanic cell type oxygen sensor and monitor whether or not the oxygen concentration has become abnormal.
However, this sensor is defective in that slight differences in electrode reaction areas occurring in producing the sensor cause differences in the outputs of the sensors. As a result, fluctuations occur in the outputs even when the sensors are produced in the same lot. As a result, the sensors are not usually replaceable for one another. Furthermore, the outputs of the sensors tend to decrease gradually as time proceeds. Accordingly, it is necessary to incorporate a calibration circuit for calibrating the fluctuation and the change with time of the output characteristics of the sensor into the oxygen concentration monitor.
Although the calibration of the differences in outputs of the sensors can be conducted by a manufacturer of the monitor, the calibration of the change with time has to be conducted by a user. Therefore, the calibration is required to be conducted as simply as possible.
Thus, various circuits have been proposed and practically used which allow a user to simply conduct such calibration.
One of the circuits is characterized by first storing an output of the sensor in fresh air as a value corresponding to the oxygen concentration of 21%, and by setting a reference value to a value obtained by subtracting a proportion of the stored output. If the oxygen concentration in the room to be measured is decreased, and the output of the sensor becomes lower than the reference value, an alarm is activated.
In the circuit as mentioned above, a calibration timing or the time for storing the fresh-air output of the sensor may be at the same time when a replaced sensor is restarted, or may be at an arbitrary time for calibration against the change in the output with elapsed time. That is, a user can calibrate by giving a contact signal to the circuit manually at the above-mentioned timing.
However, although the prior art circuits allow the user to simply conduct the calibration, they still exhibit the problem that proper calibration timing cannot be guaranteed. Because the user cannot check whether or not the oxygen concentration at the calibration time is truly 21%, the user might actually calibrate at lower oxygen concentration than 21%.
If the circuit is installed so as to indicate the alarm when the output of the sensor is decreased to the value corresponding to the proportion of 18/21 with reference to the stored value, the alarm is indicated at an oxygen concentration of 18% in a normal use. However, if the user calibrates erroneously at the oxygen concentration of 19%, the alarm is not indicated until the oxygen concentration reaches 16.3% (19.times.18/21).
Thus, if the calibration is erroneously conducted, the function of the alarm is insignificant. Additionally, if the user relies on such a monitor, he may be endangered.